1. Field of the Invention
The present invention relates to a charge acceptance limit determination apparatus and a charge acceptance limit determination method.
2. Background of the Invention
Japanese Laid-Open Patent Publication No. 2001-16800 discloses a technique of preventing unnecessary power generation by controlling power generation of an alternator based on a state of charge (SOC) of a secondary battery.
According to the aforementioned control based on the SOC, the control is generally performed in such a manner that the SOC, which is being measured, falls in a predetermined range between SOC2 and SOC1, as shown in FIG. 11. However, it is known that, when a full charge state of a secondary battery when new is defined as SOC=100%, the SOC thus defined changes with an elapse of time (a value decreases). That is to say, as shown in FIG. 12A, an SOC at an upper chargeable limit of the secondary battery, i.e., a charge acceptance limit SOC, decreases as the secondary battery degrades. FIGS. 12A to 12C are diagrams showing aging of the charge acceptance limit SOC of the secondary battery. In these drawings, references numerals 51 and 52 indicate the charge acceptance limit SOC and a chargeable charge control range, respectively.
A decrease in the charge acceptance limit SOC is caused by sulfation (crystals of sulfate) which is built up on electrode plates of the battery and which does not dissolve by being hardened during a long time of use. As the sulfation which does not dissolve increases on the electrode plates, an area of charge/discharge on the electrode plates decreases and leads to a decrease in the charge acceptance limit SOC.
When the charge acceptance limit SOC decreases as shown in FIG. 12A, the secondary battery cannot be charged to an upper limit SOC1 of a charge control range set in advance, and a charge acceptance limit SOC 51 which is lower than that will become an upper chargeable limit. As a result, the charge control range 52 will be greater than or equal to the lower limit SOC2 and less than or equal to the charge acceptance limit SOC 51, and thus the controllable range becomes narrower.
As the degradation of the secondary battery further progresses, as shown in FIG. 12B, the charge acceptance limit SOC 51 becomes lower than the lower limit SOC2 of the charge control range. In such a state, the charge control range cannot be ensured and the charging of the secondary battery will be always carried out. As a result, fuel consumption of the engine cannot be improved. In order to ensure the charging control range and to carry out charging control even in a state where degradation of the secondary battery has progressed, it is necessary to detect the charge acceptance limit SOC 51 which decreases as the degradation progresses and, based on this, to appropriately adjust the charge control range as shown in FIG. 12C.
The present invention has been contrived to solve the aforementioned problem and an object of the present invention is to provide a charge acceptance limit determination apparatus and a charge acceptance limit determination method for a secondary battery capable of accurately determining the charge acceptance limit of the secondary battery.